In Italian patent applications Nos. 27,243 A/78 and 24,668 A/79 and corresponding U.S. Pat. No. 4,278,586, there have been described composite materials consisting of:
(a) 50-90% by weight of a crystalline polyolefin, and PA1 (b) 50-10% by weight of glass fibers, said materials being characterized in that they contain, as coupling agent, from 0.5% to 5% by weight, with respect to the polyolefin, of a bis-maleamic acid of general formula: ##STR1## wherein X is a group (CHR).sub.n in which n is an integer from 2 to 18 and R is hydrogen or an alkyl with 1-6 carbon atoms, or X is a bivalent cycloaliphatic or aromatic group also having several nuclei, or a heterocyclic group; R' is hydrogen, the two R's together being also capable of forming a group (CHR).sub.n when X is (CHR).sub.n.
The preparation of such composite materials was achieved by heating the blend of polyolefin, bis-maleamic acid and glass fibers previously subjected to a finishing treatment with a silane and with a film-forming agent, preferably of polyolefinic kind having, in the molten state, a surface tension near that of the olefin polymer to be reinforced.